HCN synthesis from methane and ammonia:: mechanisms of Pt+-mediated C-N coupling

The Pt+-mediated coupling of methane and ammonia has been studied both experimentally and computationally. This system serves as a model for the Degussa process for the industrial production of the valuable feedstock hydrogen cyanide. Mass spectrometric studies demonstrate that C-N bond formation is catalyzed efficiently by Pt+. Details of the experimentally observed reaction channels have been explored computationally using the B3LYP hybrid DFT/HF functional. In the first reaction step, Pt+ dehydrogenates CH4 to yield PtCH2+; in contrast, dehydrogenation of ammonia by Pt+ is endothermic and does not occur experimentally. Starting from PtCH2+ and NH3, C-N bond formation, which constitutes the crucial step in making HCN from CH4 and NH3, is achieved via two independent pathways. The major pathway is found to be exothermic by 23 kcal mol(-1) and yields neutral PtH and CH2NH2+. The second pathway involves a dehydrogenation to yield the aminocarbene complex PtC(H)NH2+ (Delta(r)H = -36 kcal mol(-1)); dehydrogenation of PtC(H)NH2+ to PtCNH+ is exothermic with respect to PtCH2+ + NH3 (Delta(r)H = -8 kcal mol(-1)) but hindered by kinetic barriers. A comparison of Pt+ with other transition metal cations (Fe+, Co+, Rh+, W+, Os+, Ir+, and Au+) shows that Pt+ is unique with respect to its ability to activate 1 equiv of CH4 and to mediate C-N bond coupling.